That's the impression they give, but maybe not necessarily. The Lv60-70 SCH quests (which deal with some characters who are actually Nymian and had been in stasis since the War of the Magi) and talk about Setoto's father who had to "infiltrate" Amdapor seeking a cure for a sickness, and was caught and interrogated by the Amdapori. We don't know what happened to him after that, but Setoto believe it likely that they killed him.
I figure the Source's choice of "astral" and "umbral" is arbitrary - they just picked two opposing concepts to name the active and passive energy poles.
The Norvrandtian logic for naming active=Dark and passive=Light is also pretty arbitrary, except they coincidentally got it correct rather than back-to-front.
It gets confusing because there are two different uses of "Darkness" and "Light" now. There are the actual forces/energies that we're dealing with at a large scale, which is what Koji is talking about in that interview (the only concept we had at that time).
Now Shadowbringers has thrown in Darkness and Light as Norvrandt's terms for the active and passive energy poles, which coincidentally line up with the aspects of actual Darkness and Light.
You're misremembering. They do question if we got it wrong, but the reasoning is different. Key quotes from his explanation:
The people of Norvrandt associate "the pole aligned with activity and growth" (Eorzean astral) to Darkness because "life's myriad colours combine to produce black".
"The pole aligned with passivity" (Eorzean umbral) is associated with Light because "peace and tranquility [are] as purest white unmarred by colour".
The conclusions are correct, but the logic seems very arbitrary.